Patent Document 1 discloses a drive device that performs pulse shift to measure a phase voltage at the time of driving a brushless motor at extremely low rotational speed by low-speed sensorless control. In pulse shift control, it is necessary to limit the minimum value of a voltage pulse width in order to obtain a pulse induced voltage of a non-energized phase after convergence of ringing (voltage vibration) due to on/off operation of switching elements in a motor drive circuit.
Thus, for example, in Patent Document 2, two consecutive periods of a pulse width modulation (PWM) signal are divided into a major period and an adjustment period. Then, the major period is limited to a lower limit duty cycle, as a period for measuring a pulse induced voltage, and the adjustment period is set so that a duty cycle obtained by averaging the major period duty cycle and the adjustment period duty cycle becomes a set duty cycle.